When actor Con O’Neill got a call from his agent about a role in the new production of Annie Baker’s play Circle Mirror Transformation, to be directed by Bijan Sheibani, he leapt at the chance.

“It’s been a while since I’ve been on stage and I was eager to do a play but I wanted to do something more collaborative,” O’Neill says. “I’d seen Bijan’s work on a play called Barber Shop Chronicles, which was my favourite show of last year and exactly the sort of theatre I wanted to do.”

Then he asked his agent what Circle Mirror Transformation was actually about. “She tried, but nobody can explain this play! Then I got the script and, well, it’s about inheriting rather than demonstrating. It’s about being rather than pretending. I’m having more sleepless nights over this than I have about any other job. But that’s a good thing.”

Superficially, the play is about five people forming a drama therapy group in Vermont, each for very different reasons, and how this impacts upon them. O’Neill plays Schultz, a rudderless middle-aged man recently divorced from his childhood sweetheart. “He’s lovely,” O’Neill says. “I feel very attached to him. If I wasn’t who I am, he’s probably who I would have been and suddenly he doesn’t know what to do with himself anymore. What do you do when your life turns out to be a mistake? I admire him, because he doesn’t sit and drink. He goes out there and he tries to do things that will frighten him and stretch him.”

O’Neill got his first big acting break in 1983, courtesy of writer/director Bob Carlton who cast him in the Bill Shankly play You’ll Never Work Alone at the Liverpool Everyman. Since then he’s balanced theatre, film and TV work, becoming a familiar face in popular shows such as Happy Valley, Uncle, Ordinary Lies and Russell T Davies’ Manchester-made Cucumber, which he says was “the most fun I’ve ever had with my clothes on or off”.

He’s quick to spurn the received wisdom that TV acting is in some way a lesser craft than theatre work.

“I think television is just as relevant as theatre. It depends what type of telly you do, obviously. I’ve been really lucky, I’ve worked with some great writers – Russell, Sally Wainwright, Danny Brocklehurst. But I have to prepare exactly as I would for a theatre play, only without the rehearsals. Now and again, though, I just want to be in a room with other actors, a writer and a director exploring a text, and that can only happen in theatre.”

Circle Mirror Transformation is proving to be a proper challenge for O’Neill, calling for a very distinctive acting style. “It’s something that I’ve never done before,” he says. “It’s entirely naturalistic. All my old tricks have been taken away. None of us are being allowed to act. That’s why it’s terrifying. The stage is surrounded by mirrors, too, so there’s nowhere to hide. If one peg is out of place, it falls to pieces. If we get it wrong it will just be dull, but if we get it right it will be an absolute joy. That’s the thrill, really.”